Method for determining parameters of an ofdm signal and associated receiver

ABSTRACT

The invention relates to a method of determining parameters of a signal of OFDM type in a receiver comprising a tuner and a demodulator, as well as such a receiver.  
     The method comprises the steps:  
     (a) programming the demodulator with a particular value of the FFT mode parameter;  
     (b) for this particular value of FFT mode parameter and a particular value of the guard interval, exhaustive variation of other parameters of the signal;  
     (c) verification of the absence of lock-on, even temporary, of the carrier recovery loop during the phase of variation;  
     (d) in case of absence of lock-on, repetition of the previous steps with another FFT mode;  
     (e) storage of the configurations of parameters corresponding to temporary lock-ons; and  
     (f) variation of the guard interval value for each of these configurations, until permanent lock-on of the carrier recovery loop is detected.

[0001] The subject of the invention is a method for determiningparameters of a signal of OFDM type in a receiver comprising a tuner anda demodulator. The invention applies in particular within the frameworkof an over-the-air/terrestrial broadcast digital television system.

[0002] In a terrestrial digital television receiver, in particular ofDVB-T type, the circuits of the input stage must, for a given receptionfrequency, determine a certain number of parameters of the incomingsignal. Among these parameters are, in particular:

[0003] the spectral inversion (or non-inversion),

[0004] the number of carriers in a COFDM symbol,

[0005] the duration of the guard interval between symbols, and

[0006] the frequency shift (“offset” used by the programme broadcastersto distance the digital channels from adjacent analogue carriers).

[0007] According the DVB-T standard, the number of possible values ofeach of these parameters are respectively two, two, four and three. Thereceiver must thus test a maximum of 48 combinations of parameters, thiscorresponding on average to a success after 24 tries.

[0008] The subject of the invention is a method of determiningparameters of a signal of OFDM type in a receiver comprising a tuner anda demodulator, characterized in that it comprises the steps of:

[0009] (a) programming the demodulator with a particular value of theFFT mode parameter;

[0010] (b) for this particular value of FFT mode parameter and aparticular value of the guard interval, exhaustive variation of otherparameters of the signal;

[0011] (c) verification of the absence of lock-on, even temporary, ofthe carrier recovery loop during the phase of variation;

[0012] (d) in case of absence of lock-on, repetition of the previoussteps with another FFT mode.

[0013] It has been discovered that it is possible to detect the FFT mode(i.e. the number of carriers of an OFDM symbol), whatever the guardinterval, by observing the state of the demodulator. The maximum numberof configurations to be tested is thus reduced from 48 to 30, thiscorresponding on average to finding the right configuration in 15 tries.

[0014] Another subject of the invention is a receiver deviceimplementing the detection method.

[0015] Other characteristics and advantages of the invention will becomeapparent through the description of a particular exemplary embodiment,illustrated by the appended figures in which:

[0016]FIG. 1a and 1 b represent a block diagram representing a tuner aswell as a demodulator of a receiver in accordance with the invention,

[0017]FIG. 2 is a flow chart of the main part (‘Pgm1’) of the method ofdetermining parameters according to the exemplary embodiment,

[0018]FIG. 3 is a flow chart of a part (‘Pgm2’) of the method of FIG. 2,which part is intended for varying the values of certain parameters andfor verifying the at least potential lock-on for these combinations,

[0019]FIG. 4 is a flow chart of a part of the method (‘Pgm3’) of FIG. 3,which part is intended for verifying the lock-on at least of the inputstage for a given combination of parameters,

[0020]FIG. 5 is a flow chart of a part (‘Pgm4’) of the flow chart ofFIG. 4, which part is intended for verifying the lock-on of thedemodulator.

[0021] The tuner 1 of the receiver/decoder of FIGS. 1a and 1 b comprisesan input for a radio frequency (RF) signal, linked to a source 2.According to the present example, the environment in which thereceiver/decoder is operating complies with the DVB-T standard (EuropeanStandard EN 300 744 published by the ETSI), and the source in this caseis a terrestrial antenna. The input of the RF signal is linked to anamplifier 3, whose output is connected to a dual frequency conversioncircuit 4. This circuit transposes the signal to a first intermediatefrequency (signal IF1) by virtue of a first mixer 5 linked to a firstoscillator (not illustrated), then to a second intermediate frequency(signal IF2) by virtue of a second mixer 6 linked to a second oscillator(likewise not illustrated). The signal IF1 is filtered by theintermediary of a surface wave filter 7 before the second transposition.The frequency conversion circuit furthermore comprises amplifiersreferenced 8 and 9 respectively at input and output.

[0022] An automatic gain control (AGC) signal originating from thedemodulator 11 adjusts in a manner known per se the input gain of thetuner so as to obtain an amplitude of the demodulator input signalcompatible with the conversion span of the analogue/digital converter 12of the demodulator.

[0023] Lastly, the tuner 1 comprises an interface 10 with an I2C bus ofthe decoder. This bus is linked to a microprocessor (not illustrated) ofthe receiver/decoder, and makes it possible to control the operation ofthe tuner by virtue of appropriate software.

[0024] The converter 12 of the demodulator is linked to an externaloscillator 13. In a manner known per se, an AGC circuit 14 derives anappropriate gain control signal from the signals at the output of theconverter. The digitized signal (still real at this level) is convertedto complex form by a circuit 15, with the aim of preparing for the fastFourier transform. An interpolator filter 16 resamples the signal withthe aim of correcting certain clock drifts. A time synchronizationcircuit 35 controls the interpolation filter and determines the start ofthe window of the fast Fourier transform carried out by the FFT circuit18, this window corresponding to an OFDM symbol. A frequency correctionis carried out by a transposition circuit 17 before the switch to thefrequency domain. This circuit is controlled by a frequencysynchronization circuit 19 which analyses the signal in the frequencydomain at the output of the FFT circuit 18.

[0025] Certain carriers of the OFDM symbol have a predefined frequencyand a predefined power and serve to correct the common phase errors(CPE) of the signal (circuit 20). An estimation of the transmissionchannel and a corresponding equalization are carried out by theequalizer 21. The complex samples emanating from the equalizer are thenprocessed by a circuit 22 to determine in a manner known per se decisionparameters for the Viterbi decoder proper (see below). The samples arethen quantized (circuit 23), deinterleaved (circuit 24), demultiplexed(circuit 25) in the format of the Viterbi decoder (circuit 26). Thecorrected data are analysed (reference 27) with the aim of determiningthe start of a Reed Solomon block, then the external deinterleaving iscarried out (reference 28) before the Reed Solomon decoding proper(reference 29). The resulting data are descrambled (reference 30), thereverse process having been performed at the level of the transmitterfor reasons of dispersion of the energy of the signal. Lastly, a section31 makes it possible to obtain the data either in serial form, or inparallel form. The demodulator moreover possesses an interface for I2Cbus.

[0026] The demodulator is for example the L64781 circuit from LSI Logic.

[0027] The decoder receiver moreover possesses a TPS decoder 32 and anI2C bus interface 33, linking the tuner, the demodulator and amicroprocessor 34.

[0028] The TPS decoder recovers the information carried by certaincarriers of the signal received. This information, defined in theabovecited DVB-T document, moreover comprises: the FFT mode (i.e. 2K or8K), the modulation used (QPSK, 16-QAM or 64-QAM), whether the data arecoded in normal or hierarchical mode with an additional parameter (‘α’),the duration of the guard interval ({fraction (1/32)}, {fraction(1/16)}, ⅛, ¼), as well as the Viterbi puncturing rate (½, ⅔, ¾, ⅚, ⅞).This information can be recovered by the microprocessor 34. However, theTPS data cannot be extracted from the signal by the demodulator untilthe demodulator already possesses a certain number of parametersrelating to the signal. The minimum parameters required by thedemodulator to carry out lock-on of the signal for recovery of the TPSdata are:

[0029] (a) the duration of the guard interval,

[0030] (b) the FFT mode,

[0031] (c) the spectral inversion or non-inversion

[0032] (d) any frequency shift (“offset”) introduced by the transmitter.

[0033] Given that the position of the TPS carriers and the modulationemployed for the TPS data are fixed and known in advance, these data maythen be recovered without any problem.

[0034] The microprocessor must therefore determine the parameters (a) to(d). It is moreover assumed that the width of a channel is known inadvance. According to the present example, it is fixed at 8 MHz.

[0035] One of the two FFT modes is chosen, and the demodulator isprogrammed accordingly. When the chosen FFT mode is not the mode of thesignal received, then no lock-on of the carrier recovery loop (or ‘CTLstanding for “Carrier Tracking Loop”) of the demodulator is possible. Onthe other hand, it has been discovered that when the right FFT mode ischosen, the demodulator will lock-on for certain configurations of thespectral inversion and frequency shift parameters, doing so whatever thevalue of the guard interval. It is this particular feature which makesit possible to reduce the number of total configurations to be tested.

[0036] The choice of the initial FFT mode for initiating the method maybe arbitrary, or may be based on a statistical analysis of past lock-onsin one mode or in the other.

[0037] The following variables will in particular be employed in thedescription of the method:

[0038] A variable called ‘State_Latching’ is used to indicate whether aconfiguration of parameters allows latching or otherwise of the inputstage, or whether the configuration could potentially give rise tolatching, if the guard interval were chosen correctly. This firstvariable may have one of the three values: ‘Absence_Carrier’,‘Potential_Configuration_Detected’, ‘Detection_Carrier’.

[0039] A second variable called ‘Potentials_Combination’ gives, for agiven FFT mode, the number of configurations of the (Spectral inversion,Frequency shift) pair potentially able to correspond to theconfiguration necessary for latching, following the analysis of thebehaviour of the demodulator carrier recovery loop.

[0040] Main method (‘Pgm1’):

[0041]FIG. 2 is a flow chart of the general method for determining theoperating parameters (‘Pgm1’). An initialization is firstly performed.The latter comprises the choosing of one of the two FFT modes, of aguard interval, of an inversion, of a shift, as well as of values foreach of the other parameters (modulation, puncturing rate, etc). Thedemodulator circuit is programmed accordingly by the microprocessor.

[0042] For the chosen FFT mode, the routine Pgm2, seen in greater detailin relation to FIG. 3, reviews the various configurations of values ofthe (Spectral inversion, Frequency shift) pair and for eachconfiguration determines whether there is lock-on or at least potentiallock-on:

[0043] (a) If there is lock-on, signifying that the FFT mode and theguard interval which were initially chosen were correct, then the valuereturned for the variable ‘State_Latching’ is ‘Detection_Carrier’, thenumber of potential configurations is then irrelevant.

[0044] (b) If there is no lock-on, because the guard interval is notcorrect, the FFT mode being correct, the returned number of potentialconfigurations will be non-zero and the value of the variable‘State_Latching’ will be ‘Absence_Carrier’.

[0045] (c) If there is no lock-on because the FFT mode is not correct,then the value of the variable ‘State_Latching’ will be‘Absence_Carrier’ and the number of potential configurations will bezero.

[0046] On the basis of the above result, the method of FIG. 2 determinesthe course to be followed. In case (c), since the FFT mode initiallychosen does not seem to be the right one, the entire procedure is begunagain with the other FFT mode. Following this, one of cases (a) or (b)normally holds.

[0047] If case (a) holds, then there was lock-on and the correctconfiguration has been stored. The procedure is then terminated.

[0048] If case (b) holds, the guard interval is varied, and for eachvalue of guard interval, latching is tested for one of the potentialconfigurations previously stored, by way of the routine Pgm3 of FIG. 4.

[0049] To summarize, configurations are tested firstly while ignoringthe guard interval and then, if necessary, the potential configurationsare revisited while varying this interval.

[0050] Routine for varying the frequency shift and inversion parameters(‘Pgm2’):

[0051]FIG. 3 represents the routine performing the latching tests forall the values of the (Inversion, Frequency Shift) parameter pair. Foreach pair of values, one determines whether there is latching orpotential latching.

[0052] The FFT mode and guard interval parameters are not modified bythis routine.

[0053] Firstly, a check verifies whether all the configurations of thepair have been tested and whether the latching state is indeed‘Absence_Carrier’. Specifically, if a latching ought to have taken placeduring one of the tests, then the loop would be exited forthwith.

[0054] The test loop itself begins by calling the routine ‘Pgm3’, whichverifies lock-on of the CTL loop, then, if such lock-on is detected, thelock-on of the error correction part of the input stage. ‘Pgm3’ returnsthe value of the variable ‘State_Latching’ for a given configuration.

[0055] On the basis of the latching state returned by Pgm3, Pgm2 savesthe configuration of the parameters if State_Latching indicates thatthis configuration will potentially give lock-on. In such a case,State_Latching is reset to the value ‘Absence_Carrier’, so that the mainloop of Pgm2 continues to trawl through the various configurations.

[0056] Routine for detecting lock-on of the carrier recovery loop CTLand of the error correction part of the demodulation circuit (‘Pgm3’):

[0057] Firstly, the microprocessor reinitializes all the recovery loopsand internal handlers of the terrestrial demodulator. A new routine(‘Pgm4’) is then initiated. The latter determines, by virtue of certainstate registers of the demodulation circuit, the state of the latchingof the CTL part and returns the corresponding state of State_Latching.If a carrier has been detected at this level by Pgm4, then the routinePgm3 pushes the investigation further, by activating the mode ofautomatic demodulation of the TPS carriers. This mode makes it possibleto programme the demodulator with the data thus recovered. Once thisautomatic mode has been initiated, the microprocessor scans a stateregister of the demodulation circuit so as to verify that thesynchronizations of the Reed-Solomon decoder 29 and of the descrambler30 are effective. This verification is performed for a given time. Thelatching state is determined from this verification (‘Presence_Carrier’if there is dual synchronization, ‘Absence_Carrier’ otherwise).

[0058] If there has been synchronization, the routine Pgm3 immediatelyreturns this information to the routine Pgm2, in the form of the state‘Presence_Carrier’. If there has not been synchronization at the levelof the error correction part, the state returned is finally‘Potential_Configuration’. The reason for this is that Pgm3 wasinitiated by Pgm2 following lock-on of the carrier detection part of thedemodulator. Moreover, in the latter case, the TPS auto-detection stopssince the configuration, although indicating that certain parametershave the right values, is not the sought-after final configuration.

[0059] Routine for detecting lock-on of the carrier recovery loop CTL(‘Pgm4’).

[0060] The flow chart of this routine is given by FIG. 5. The routinecomprises a loop which is run through as long as a carrier has not beendetected, at least sporadically, for a maximum waiting time of DeltaT2

[0061] This detection is performed in the following manner: themicroprocessor monitors the state of one of the registers of thedemodulator circuit to determine whether on the one hand there issynchronization of the demodulator on the COFDM frame, and whether onthe other hand, there is, at the moment of the test, latching of thecarrier recovery loop (also called the Carrier Tracking Loop). In thecase of the component used in the present example, the registerconcerned is the ‘Performance Monitoring Register 1’, located at theaddress 0×32, the two monitored bits being bits D2 and D3.

[0062] Detection is validated only if the synchronization of thedemodulator on the TPS frame and the recovery of the carrier are validsimultaneously. In the converse case, the number of times the testrelating to the CTL bit is positive consecutively for the intervalduration DeltaT2 is counted up. Ultimately, the potential detection willbe established if this counter is greater than or equal to a limit N,which is for example taken equal to 6.

[0063] The routine then returns the value of the variable State_Latched:i.e. ‘Absence_Carrier’ if the above criteria are not fulfilled, or‘Potential_Configuration’ if these criteria are fulfilled.

[0064] It is clear that the invention applies also within frameworksother than that of DVB-T: for example in the case of a system where morethan two FFT modes are used.

1. Method of determining parameters of a signal of OFDM type in areceiver comprising a tuner and a demodulator, characterized in that itcomprises the steps of: (a) programming the demodulator with aparticular value of the FFT mode parameter; (b) for this particularvalue of FFT mode parameter and a particular value of the guardinterval, exhaustive variation of other parameters of the signal; (c)verification of the absence of lock-on, even temporary, of the carrierrecovery loop during the phase of variation; (d) in case of absence oflock-on, repetition of the previous steps with another FFT mode. 2.Method according to claim 1, characterized in that the parameters whosevalues are varied in step (c) are the inversion and the frequency shift.3. Method according to one of claims 1 and 2, characterized in that itfurthermore comprises the steps of: (e) storage of the configurations ofparameters corresponding to temporary lock-ons, and (f) variation of theguard interval value for each of these configurations, until permanentlock-on of the carrier recovery loop is detected.
 4. Method according toclaim 3, characterized in that it furthermore comprises the step ofverification, for a configuration giving rise to a permanent lock-on ofthe carrier recovery loop, of the synchronization of at least one errorcorrection module of the input stage of the receiver.
 5. Methodaccording to one of claims 1 to 4, characterized in that a detection oflock-on of the carrier recovery loop is carried out by the monitoring ofstate bits of the demodulator.
 6. Device for receiving a signal of OFDMtype, the said device comprising a tuner and a demodulator which islinked to the tuner, characterized in that it furthermore comprises:means (33, 34) of programming the demodulator for a particular value ofan FFT mode parameter of the signal; means (34) of exhaustive variationof other parameters of the signal, for this particular value of FFT modeparameter and a particular value of the guard interval; means (34) ofverification of the absence of lock-on, even temporary, of the carrierrecovery loop during the phase of variation and, in case of absence oflock-on, for repeating this process with another FFT mode.